Thermal and Geochemical Anomalies in the Eastern Snake River Plain Aquifer: Contributions to a Conceptual Model of the Proposed FORGE Test Site

Data from the U.S. Geological Survey’s National Water Information System (NWIS) database reveal the existence of a number of thermally anomalous areas on the eastern Snake River Plain (ESRP) aquifer, most of them near its margins, and NWIS temperature and chemistry data provided conclusive evidence that thermal waters originating in the hot rhyolitic rocks underlying the ESRP basalts inject heat and solute mass into the overlying ESRP aquifer. Thermal waters issuing from hot felsic basement rocks in southern Idaho are Na-HCO3 type due to water-rock reactions at elevated T. They are characteristically depleted in Ca and Mg, typically low in Cl (depending on the source water that reacts with the felsic rock), and high in pH, Na, HCO3, SiO2, Li and B (as well as F, if not diluted with high-Ca waters). These diagnostic tracers are seen in thermal waters (≤44 C) that issue from shallow wells in late Neogene rhyolites of the Newdale thermal area and thermal waters (≤57 C) sampled from INEL-1, a 3.2 km-deep borehole in rhyodacite and welded tuff. A correlation between SiO2 and Na/Ca molar ratio appears to be a sensitive indicator of the presence of rhyolite-labeled water, as seen in thermal waters of the Newdale area, the highest-temperature anomaly in ground waters associated with ESRP volcanic rocks, and in dilute mixtures in the ESRP aquifer adjacent to the Newdale area, as well as in warm ground waters of the ESRP aquifer near the INL. On the INL, contamination from anthropogenic sources and from recharge of both cold tributary surface and ground waters along the aquifer’s northern margin renders geochemical tracing of thermal water ineffective in the area of the proposed FORGE test site. The clearest evidence of thermal water entering the aquifer through its base is seen 10-20 km to the southeast where previously identified thermal and geochemical anomalies have been identified, characterized by elevated Na, SiO2, Li, B and slight enrichments in F, as well as high dissolved He. An estimate of the advective flux of thermal water in this area was derived via a two-component mixing model. Based on published aquifer flux and porosity information, however, the magnitude of the estimated thermal flux through the base of the aquifer is incompatible with core-scale data on the rhyolite’s hydraulic conductivity and vertical gradient observed beneath the INEL-1. This suggests that advective heat transport from the rhyolite is not spatially uniform but focused along localized preferential flow paths within the rhyolite and the mineralized basalts that overlie the rhyolite. This interpretation is consistent with observations of thermal fluid flow in INEL-1’s fractured rhyolitic rocks and supports the hypothesis that the rhyolitic basement hosts preferential flow paths. These findings were incorporated in a geohydrologic conceptual model of the proposed INL FORGE field site.